Bootstrap4 refactor

[fedetask]

Using Bootstrap4 and refactoring the website according to #44

• [X] Add Bootstrap4
• [X] Create layouts for content in a main column, with left-right spacing and annotations
• [X] Refactor cards
• [x] Finish refactoring papers
• [x] Refactor lectures
• [x] Refactor experiments
• [x] Improve main page
• [x] Create ML page
• [ ] Refactor comment form

[OleguerCanal]

Looks very nice! I'll add some todos here so we can keep track of what we do

• [x] Use Bold Roboto typeface for menu bar, titles, section headers
• [x] Use Roboto typeface for figure description and side-annotations
• [x] Use Merriweather / Open Sans typeface for main text
• [x] Top header not following when scrolling
• [x] I think a thinner header might look nicer
• [x] Also I've seen OpenAI one is actually white, which looks nice, so maybe we could go for that.
• [ ] Could we highlight the page we are as they do?
• [ ] Use narrower middle text
• [x] When using half-screen window or phone, could we hide the side-index?
• [x] I like the purple color in the index but we only use it there to highlight, so i think we should either use it as a highlighting color in more places or just use black to highlight there (i would go for black for now))
• [x] Are figures centered? It doesnt feel they are anymore
• [x] Figures in RL page cards not centered

Idea is to have something similar to this

[OleguerCanal]

The title bar looks a bit weird with 2-line things when using half screen. What do you think of just using ML, RL, Papers, Experiments? @fedetask

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